The present invention relates to equipment and methods for efficiently generating fluid streams that may be used in the manufacture of microelectronic devices. More specifically, the present invention relates to equipment and methods in which process fluids are dispensed through a nozzle including a plurality of divergent orifices. The invention is particularly useful for efficiently generating a cryogenic aerosol.
The manufacture of microelectronic devices is complex and involves several process steps. Many of these steps involve applying one or more streams of fluid and/or particles (e.g., cryogenic cleaning crystals) onto a workpiece surface in order to carry out one or more of etching, cleaning, stripping, rinsing, and the like. Often, it may be important to ensure that the fluid is directed onto the workpiece surface as efficiently as possible to help optimize process performance. Cryogenic-aerosol cleaning of microelectronic workpieces is one illustrative area of concern.
Cryogenic-aerosol cleaning of wafers is a xe2x80x9cdryxe2x80x9d cleaning alternative to more conventional wet chemical cleans for particle and film residue removal. This technique is of particular interest in back-end-of-line applications in which wet chemicals might potentially corrode device features such as metal lines. Cryogenic-aerosol cleaning is able to use non-corrosive, inert substances such as argon, nitrogen, carbon dioxide, and/or the like, as the cleaning fluid. The cleaning mechanisms are mechanical rather than chemical. The process is therefore environmentally friendly and device-safe.
A schematic drawing of an illustrative cryogenic cleaning process is shown in FIG. 1. A cryogenic apparatus 1 includes a chuck (not shown) that supports a microelectronic workpiece 1a. A nozzle 2 extends across workpiece 1a. An array of cryogenic aerosol jets 3 issue from a plurality of nozzle orifices (not shown) of nozzle 2. The jets 3 include aerosol crystals 3a. These jets 3 of particles 3a impinge upon the surface of workpiece 1a, dislodging contaminants 4 adhering to the workpiece surface. The surface may be flat or patterned. Typical contaminants 4 might include one or more of film or particle residue generated as a result of etching and ashing processes. Nozzle 2 and the chuck move relative to each other to ensure that the jets clean the entirety of surface of workpiece 1a. In the particular approach of FIG. 1, nozzle 2 is stationary, while the chuck, and hence workpiece 1a, move so that the entire surface of workpiece 1a is cleaned by movement underneath nozzle 2. Of course, in other embodiments, the chuck could be stationary while nozzle 2 moves. In other embodiments, suitable relative movement may be obtained if both the chuck and nozzle 2 move relative to each other. For example, U.S. Pat. No. 5,942,037 describes a system that has a movable chuck and a rotatable and translatable nozzle.
The formation of cryogenic aerosol jets is an interesting process. The jets are formed by the rapid expansion of a suitable cryogenic fluid through the array of nozzle orifices. The fluid is generally a liquid, gas, or mixture thereof of one or more materials such as nitrogen, argon, carbon dioxide, water, or mixtures of these. The fluid enters the nozzle orifices at one pressure (e.g, 80 psia) and exits into a chamber maintained at a considerably lower pressure (typically below 1 psia). The resultant expansion of the fluid results in the formation of solid, or solid-liquid, particle clusters in each jet due to evaporative cooling. Further discussion of aerosol formation mechanisms can be found in U.S. Pat. No. 5,942,037; N. Narayanswami, xe2x80x9cA Theoretical Analysis of Wafer Cleaning Using a Cryogenic Aerosol,xe2x80x9d J. of the Electrochemical Society, 146-2:767-774 (1999); N. Narayanswami et al., xe2x80x9cDevelopment and Optimization of a Cryogenic Aerosol Based Wafer Cleaning System,xe2x80x9d 28th Annual Meeting of the Fine Particle Society Proceedings (1998); and N. Narayanswami et al, xe2x80x9cParticle Removal Mechanisms in Cryogenic-Aerosol-Based Wafer Cleaning,xe2x80x9d FSI International Document No. 1133-TRS-0499 (1999).
U.S. Pat. Nos. 4,747,421 and 4,806,171 describe an apparatus for cleaning substrates using CO2 aerosol particles. U.S. Pat. Nos. 4,974,375 and 5,009,240 describe sandblasting devices that use ice particles generated using water. U.S. Pat. Nos. 5,062,898, and 5,209,028, and 5,294,261 describe the use of cryogenic aerosols for surface cleaning. Borden et. al., in a paper presented at the Ultraclean Manufacturing Conference, pp. 55-60, October 1994, describes the use of CO2 snow jet spray in silicon wafer cleaning.
Contaminant particle removal efficiency is an important criterion for assessing the performance of a cryogenic cleaning process. Contaminant particle removal efficiency refers to the percentage of particles that are removed as a result of cryogenic aerosol cleaning. Particle removal efficiency characteristics are often viewed separately for particles within different size ranges, because it is important that particle removal efficiency be as high as possible for both large and small particles. Given the increasing miniaturization of microelectronic structures, even the presence of very small particles can significantly impair or ruin the performance of the resultant microelectronic device. Unfortunately, although cryogenic cleaning removes relatively large particles easily, smaller particles are much harder to remove. As a consequence, particle removal efficiency has been observed to decrease with decreasing particle size.
For example, in one illustrative test, about 36,000 particles were dunk deposited onto the surface of a test wafer. Cryogenic cleaning was used to clean the wafer surface, and then the particle removal efficiency was evaluated. Not surprisingly, larger particles were easily removed. Analysis showed that about 100% of particles greater in size than 0.5 micrometer were removed. Performance for smaller particles, however, was not as good. Analysis showed that only about 88% of particles greater in size than 0.1 micrometer were removed.
What is needed, therefore, is a way to improve the ability of a cryogenic aerosol to remove smaller particles.
The present invention provides an improved nozzle design that discharges more powerful, more focused streams of fluid and/or particles through a series of nozzle orifices distributed along a length of the nozzle. The present invention may be incorporated into a wide range of microelectronic device manufacturing processes and equipment types for which an array of more forceful, more focused process streams are desired for treating microelectronic workpieces.
The present invention is particularly useful to cryogenically clean microelectronic workpieces, where the improvements allow the conventionally more troublesome smaller contaminant particles to be cleaningly removed with greater particle removal efficiency. For example, whereas one conventional nozzle structure might allow 0.105 to 0.120 micrometer SiN particles to be removed with a particle removal efficiency of only 87.9%, use of the present invention boosts particle removal efficiency to at least 93.5%. As another example, one conventional nozzle structure might allow 0.064 micrometer W particles to be removed with a particle removal efficiency of only 87%, whereas the present invention boosted this value to 95.8%.
In one aspect, the present invention relates to an apparatus for cryogenically treating a surface of a workpiece. The apparatus includes a treatment chamber in which the workpiece is positioned for a cryogenic treatment, wherein the treatment chamber is at a first, relatively low pressure. The apparatus includes a nozzle that comprises a plurality of nozzle orifices that are distributed along a length of the nozzle in a manner effective to aim at an angle of impingement toward the workpiece surface. Each nozzle orifice comprises a nozzle inlet through which a process fluid enters the nozzle and an outlet through which the process fluid is cryogenically discharged from the nozzle toward the workpiece surface. The nozzle is at a second, relatively high pressure such that the difference between the first and second pressures helps to cause the process fluid to be cryogenically discharged from the the nozzle, and wherein each of at least two of the nozzle orifices comprises a divergent section. The divergent section allows the cyrogenic aerosol to expand in a relatively controlled fashion and preferably is proximal to the orifice outlet. In other embodiments, the divergent section may be proximal to the inlet, or it may extend continuously from the inlet to the outlet, or it may be positioned inside the nozzle orifice between and spaced apart from the inlet and outlet.
In another aspect, the present invention relates to a method of cryogenically treating a surface of a workpiece. A process fluid is caused to be cryogenically discharged from a plurality of orifices of a nozzle onto the workpiece surface. The orifices each have an inlet through which the fluid enters the orifice and an outlet through which the fluid is discharged from the orifice. At least two of the orifices comprises a divergent section, preferably being proximal to the orifice outlet.